If you are in the east end, there is a butcher shop in Sherwood Park who carries many British products (Butcher Block beside the Home Depot on Baseline road). If he does not have it, I suspect he can get it.

It says right on their website that they sell Bovril & Marmite when you check out the "Spreads" section description under the site map link.

Definitely call before heading in, because they have the weirdest store hours I've ever seen; only open 3 days a week from 11am to 5pm on Wed, Fri & Sat. I'll be going there this weekend, I'll report back!